The present invention relates to improvements in or relating to magnetic resonance imaging (MRI) systems, and is more particularly concerned with the construction of shimming apparatus for use therewith.
A MRI system consists of a magnet which provides a main magnetic field, a gradient coil to superimpose a linearly varying field over the main field, and an RF coil transmit/receive system which is used to receive the signals that are used to construct the image. A good base homogeneity of the main magnetic field in the system is essential for producing good quality images, and hence the process of shimming has been developed in order to establish good base homogeneity.
Shimming is the process by which the field in an MRI magnet is corrected. It is an essential activity, without which the system cannot function. Correction of magnetic field deviations can be performed using any elements that produce a magnetic field. Thus, current carrying conductors, soft iron in the presence of a magnetic field, and permanent magnets have been known to be used. For MRI magnets, passive shimming is extensively used. Passive shimming is defined as the use of magnetic material elements, be they soft iron or permanent magnets, for the correction of magnetic fields. The use of passive shimming is extensive because it is simple, cheap and can achieve high levels of homogeneity.
Passive shimming requires the placing of magnetic material in one or more positions around the bore of a magnet in order to perform the field correction. Traditionally, the process takes the form of a field map of the region of interest, normally defined as a spherical volume. The field values are then decomposed into, as is well known in the art, spherical harmonics, which describe the field variation over the volume. Thus, two elements are required to effect passive shimming, namely, the software which is used to predict the positions of the pieces of magnetic material necessary to perform the field correction, and the hardware, called the shimset, comprising pieces of magnetic material which are appropriately positioned to effect the field correction.
For normal coil wound magnets (also known as solenoidal magnets), the shimset is no more than the hardware needed to locate the precise distribution of magnetic material used to correct inhomegeneities in the magnetic field. In practice, one or more trays are distributed azimuthally over the shimset, each tray having a series of pockets in which shim iron can be placed. The quantity of shim iron in each pocket depends on the inhomogeneity of the magnetic field. Axial and azimuthal symmetry have traditionally been used in the design of shimsets and there has been no need to deviate from that.